animal development. the question of how a zygote becomes an animal has been asked for centuries as...
TRANSCRIPT
Animal Animal DevelopmentDevelopment
• The question of how a zygote becomes an animal has been asked for centuries
• As recently as the 18th century, the prevailing theory was called preformation
• Preformation is the idea that the egg or sperm contains a miniature infant, or “homunculus,” which becomes larger during development
WHAT DETERMINES DEVELOPMENT
• Development is determined by the zygote’s genome and differences between embryonic cells
• Cell differentiation is the specialization of cells in structure and function
• Morphogenesis is the process by which an animal takes shape
Big ideas
• Gametes (fertilizaiton) • Zygote (cleavage) • Blastula (gastrulation) • Gastrula (neurulation) • Organogenesis• Role of genes & protein concentration
gradients• Induction: communication from an inducer
to a competent responder
• Fertilization• 2 major events:
• Fertilization brings the haploid nuclei of sperm and egg together, forming a diploid zygote
• The sperm’s contact with the egg’s surface initiates metabolic reactions in the egg that trigger the onset of embryonic development
• Most info comes from sea urchin studies– External fertilization – Problems of external fertilization:
• Dilution/protection of gametes in the enormous volume of the ocean
• Correct species fertilization
• Blocking polyspermy
The Acrosomal Reaction
• The acrosomal reaction is triggered when the sperm meets the egg
• This reaction releases hydrolytic enzymes that digest material surrounding the egg
• Acrosomal process adheres to receptors on vitelline layer (species specific)– Sperm/egg membranes fuse, sperm nucleus
enters– Na+ influx, depolarization– Depolarization sets up fast block to polyspermy
Fast block polyspermy
Sperm-bindingreceptors
Jelly coat
Acrosome
Actin
Spermhead
Basal body(centriole)
Sperm plasmamembrane
SpermnucleusContact
Acrosomalreaction
Acrosomalprocess
Contact and fusionof sperm and eggmembranes Entry of sperm
nucleus
Cortical reaction
Fertilizationenvelope
Egg plasmamembrane
Vitelline layer
Hydrolytic enzymes
Corticalgranule
Fused plasmamembranes
Perivitellinespace
Cortical granulemembrane
EGG CYTOPLASM
The Cortical Reaction• Fusion of egg and sperm also initiates the cortical
reaction• This reaction induces a rise in Ca2+ in cytoplasm
that stimulates cortical granules to release their contents outside the egg
• Cortical granules fuse w/ membrane– Enzymes– Polysaccharides – Fertilization envelope formed = slow block to
polyspermy (follows repolarization)
• These changes cause formation of a fertilization envelope that functions as a slow block to polyspermy
Fast block polyspermy
1 sec beforefertilization
Point ofspermentry
10 sec afterfertilization
Spreading waveof calcium ions
20 sec 30 sec
500 µm
Activation of the Egg
• The sharp rise in Ca2+ in the egg’s cytosol increases the rates of cellular respiration and protein synthesis by the egg cell– Chemical signals from cortical rxn cause H+ to be
transported out --> increase in pH
• Nuclei fuse • Egg/sperm differences
– Egg contains proteins, mRNA not found in sperm– Ca2+ injection, temperature shock can cause artificial
activation
• With these rapid changes in metabolism, the egg is said to be activated
LE 47-5
Binding of sperm to egg
Acrosomal reaction: plasma membranedepolarization (fast block to polyspermy)
Increased intracellular calcium level
Cortical reaction begins (slow block to polyspermy)
Formation of fertilization envelope complete
Increased intracellular pH
Fusion of egg and sperm nuclei complete
Increased protein synthesis
Onset of DNA synthesis
First cell division
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Se
co
nd
s
2
3
68
10
4
20
30
501
2
40
34
10
5
20
3040
9060
Min
ute
s
Fertilization in Mammals
• In mammalian fertilization, the cortical reaction modifies the zona pellucida as a slow block to polyspermy
LE 47-6
Folliclecell
Acrosomalvesicle
Egg plasmamembrane
Zonapellucida Sperm
nucleus
Corticalganules
Spermbasalbody
EGG CYTOPLASM
Cleavage
• Fertilization is followed by cleavage, a period of rapid cell division without growth
• Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres
LE 47-7
Fertilized egg Four-cell stage Morula Blastula
• The eggs and zygotes of many animals, except mammals, have a definite polarity
• The polarity is defined by distribution of yolk, with the vegetal pole having the most yolk
• The development of body axes in frogs is influenced by the egg’s polarity
LE 47-8
Anterior
Right
Animal pole
Graycrescent
DorsalVentral
Left
Posterior
Body axes Establishing the axes
Futuredorsalside oftadpole
Point ofspermentry
Firstcleavage
Vegetalhemisphere Vegetal pole
Point of sperm entry
Animalhemisphere
• Cleavage planes usually follow a pattern that is relative to the zygote’s animal and vegetal poles
LE 47-9
Zygote
2-cellstageforming
8-cellstage
4-cellstageforming
Animal pole Blasto-coel
Blastula(crosssection)
Vegetal poleBlastula (at least 128 cells)
0.25 mm
Eight-cell stage (viewedfrom the animal pole)
0.25 mm
• Meroblastic cleavage, incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds
LE 47-10
Blastocoel
Fertilized egg
BLASTODERM
HypoblastEpiblastYOLK MASS
Cutaway view ofthe blastoderm
Blastoderm
Four-cell stage
Zygote
Disk ofcytoplasm
• Holoblastic cleavage, complete division of the egg, occurs in species whose eggs have little or moderate amounts of yolk, such as sea urchins and frogs
Gastrulation
• Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut
• The three layers produced by gastrulation are called embryonic germ layers– The ectoderm forms the outer layer
– The endoderm lines the digestive tract
– The mesoderm partly fills the space between the endoderm and ectoderm
Video: Sea Urchin Embryonic Development
• The mechanics of gastrulation in a frog are more complicated than in a sea urchin-INVAGINATION
• OTHERS- INVOLUTION
LE 47-12
Future ectoderm
Key
Future mesoderm
Future endoderm
Archenteron
Blastocoelremnant
Ectoderm
MesodermEndoderm
Yolk plugYolk plugGastrula
Blastocoelshrinking
Blastocoel
Dorsal tip of blastopore
CROSS SECTION
Animal pole
Dorsal lipof blastopore
Vegetal pole Blastula
SURFACE VIEW
• Gastrulation in the chick and frog is similar, with cells moving from the embryo’s surface to an interior location
• During gastrulation, some epiblast cells move toward the blastoderm’s midline and then detach and move inward toward the yolk. INVOLUTION
LE 47-13
Futureectoderm
Epiblast
Migratingcells(mesoderm)
YOLK
HypoblastEndoderm
Primitivestreak
Organogenesis
• During organogenesis, various regions of the germ layers develop into rudimentary organs organs
• Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm
Video: Frog Embryo Development
LE 47-14a
Neural folds
Neuralplate
LM1 mm
Neuralfold
Notochord
Archenteron
Neural plate formation
Endoderm
Mesoderm
Ectoderm
LE 47-14b
Neuralfold
Neural plate
Neural tube
Formation of the neural tube
Neural crest
Outer layerof ectoderm
Neural crest
The neural plate soon curves inward, forming the neural
tube
LE 47-14c
1 mm
Notochord
Archenteron(digestive cavity)
Neural tube
Neural crest
Eye Somites Tail bud
SEM
Coelom Somite
Somites
•Mesoderm lateral to the notochord
forms blocks called somites
•Lateral to the somites, the
mesoderm splits to form the coelom
LE 47-15
Notochord
ArchenteronEndoderm
Mesoderm
Ectoderm
Neural tube
Eye
Coelom
Somite
Somites
Neural tube
Lateral fold
Yolk stalkYOLK
Form extraembryonicmembranes
Yolk sac
Early organogenesis
Forebrain
Heart
Bloodvessels
Late organogenesis
•Many structures are derived from the three embryonic germ layers during organogenesis
Developmental Adaptations of Amniotes
• Embryos of birds, other reptiles, and mammals develop in a fluid-filled sac in a shell or the uterus
• Organisms with these adaptations are called amniotes
• In these organisms, the three germ layers also give rise to the four membranes that surround the embryo
LE 47-17
Embryo
Amnioticcavitywithamnioticfluid
AllantoisAmnion
Albumen
Yolk(nutrients)
Yolk sacChorion
Shell
Mammalian Development• The eggs of placental mammals
– Are small and store few nutrients– Exhibit holoblastic cleavage– Show no obvious polarity
• Gastrulation and organogenesis resemble the processes in birds and other reptiles
• Early cleavage is relatively slow in humans and other mammals
• At completion of cleavage, the blastocyst forms• The trophoblast, the outer epithelium of the
blastocyst, initiates implantation in the uterus, and the blastocyst forms a flat disk of cells
• As implantation is completed, gastrulation begins• The extraembryonic membranes begin to form• By the end of gastrulation, the embryonic germ
layers have formed-ECTODERM, MESODERM AND ENDODERM
LE 47-18a
Blastocystreaches uterus.
Endometrium(uterine lining)
Maternalbloodvessel
Blastocystimplants.
Inner cell mass
Trophoblast
Blastocoel
Hypoblast
Trophoblast
Epiblast
Expandingregion oftrophoblast
LE 47-18b
Hypoblast
Chorion (fromtrophoblast
Epiblast
Amnioticcavity
Amnion
Yolk sac (fromhypoblast)
Extraembryonic mesoderm cells(from epiblast)
Extraembryonicmembranes startto form andgastrulationbegins.
Amnion
Chorion
Endoderm
Mesoderm
Ectoderm
Yolk sac
Extraembryonicmesoderm
Gastrulation has produced a three-layered embryo with fourextraembryonic membranes.
Allantois
Expandingregion oftrophoblast
• The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and develop in a similar way
Morphogenesis in animals involves specific changes in cell shape, position, and adhesion
• Morphogenesis is a major aspect of development in plants and animals
• But only in animals does it involve the movement of cells
The Cytoskeleton, Cell Motility, and Convergent Extension
• Changes in cell shape usually involve reorganization of the cytoskeleton
• Microtubules and microfilaments affect formation of the neural tube
LE 47-19Ectoderm
Neuralplate
• The cytoskeleton also drives cell migration, or cell crawling, the active movement of cells
• In gastrulation, tissue invagination is caused by changes in cell shape and migration
• Cell crawling is involved in convergent extension, a morphogenetic movement in which cells of a tissue become narrower and longer
LE 47-20
ConvergenceExtension
Roles of the Extracellular Matrix and Cell Adhesion Molecules
• Fibers of the extracellular matrix may function as tracks, directing migrating cells along routes
• Several kinds of glycoproteins, including fibronectin, promote cell migration by providing molecular anchorage for moving cells
LE 47-21
Direction of migration50 µm
• Cell adhesion molecules contribute to cell migration and stable tissue structure
• One class of cell-to-cell adhesion molecule is the cadherins, which are important in formation of the frog blastula
LE 47-22
Control embryo
Experimental embryo
The developmental fate of cells depends on their history and on
inductive signals• Coupled with morphogenetic changes,
development requires timely differentiation of cells at specific locations
• Two general principles underlie differentiation:– During early cleavage divisions, embryonic cells
must become different from one another– After cell asymmetries are set up, interactions
among embryonic cells influence their fate, usually causing changes in gene expression
Fate Mapping
• Fate maps are general territorial diagrams of embryonic development
• Classic studies using frogs indicated that cell lineage in germ layers is traceable to blastula cells
LE 47-23a
Fate map of a frog embryo
Epidermis Centralnervoussystem
Blastula
Epidermis
Neural tube stage(transverse section)
Endoderm
Mesoderm
Notochord
• Techniques in later studies marked an individual blastomere during cleavage and followed it through development
LE 47-23b
Cell lineage analysis in a tunicate
Establishing Cellular Asymmetries
• To understand how embryonic cells acquire their fates, think about how basic axes of the embryo are established
The Axes of the Basic Body Plan
• In nonamniotic vertebrates, basic instructions for establishing the body axes are set down early, during oogenesis or fertilization
• In amniotes, local environmental differences play the major role in establishing initial differences between cells and, later, the body axes
Restriction of Cellular Potency
• In many species that have cytoplasmic determinants, only the zygote is totipotent
• That is, only the zygote can develop into all the cell types in the adult
• Unevenly distributed cytoplasmic determinants in the egg cell help establish the body axes
• These determinants set up differences in blastomeres resulting from cleavage
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Left (control): Fertilized salamander eggs were allowed to divide normally, resulting in the gray crescent being evenly divided between the two blastomeres.
Right (experimental): Fertilized eggs were constricted by a thread so that the first cleavage plane restricted the gray crescent to one blastomere.
Gray crescent
Gray crescent
Normal Bellypiece
Normal
The two blastomeres were then separated and allowed to develop.
• As embryonic development proceeds, potency of cells becomes more limited
Cell Fate Determination and Pattern Formation by Inductive
Signals
• After embryonic cell division creates cells that differ from each other, the cells begin to influence each other’s fates by induction
The “Organizer” of Spemann and Mangold
• Based on their famous experiment, Spemann and Mangold concluded that the blastopore’s dorsal lip is an organizer of the embryo
• The organizer initiates inductions that result in formation of the notochord, neural tube, and other organs
LE 47-25a
Nonpigmented gastrula(recipient embryo)
Pigmented gastrula(donor embryo)
Dorsal lip ofblastopore
LE 47-25b
Secondary (induced) embryo
Primarystructures:
Secondarystructures:
Neural tube
Notochord
Notochord (pigmented cells)
Neural tube (mostly nonpigmented cells)
Primary embryo
Formation of the Vertebrate Limb
• Inductive signals play a major role in pattern formation, development of spatial organization
• The molecular cues that control pattern formation are called positional information
• This information tells a cell where it is with respect to the body axes
• It determines how the cell and its descendents respond to future molecular signals
• The wings and legs of chicks, like all vertebrate limbs, begin as bumps of tissue called limb buds
LE 47-26a
Anterior
Organizer regions
Limb bud
PosteriorZPA
AER
50 µm
Apicalectodermalridge
• The embryonic cells in a limb bud respond to positional information indicating location along three axes
LE 47-26b
Digits
AnteriorVentral
Distal
Posterior
Proximal
Dorsal
Wing of chick embryo
• One limb-bud organizer region is the apical ectodermal ridge (AER)
• The AER is thickened ectoderm at the bud’s tip
• The second region is the zone of polarizing activity (ZPA)
• The ZPA is mesodermal tissue under the ectoderm where the posterior side of the bud is attached to the body
• Tissue transplantation experiments support the hypothesis that the ZPA produces an inductive signal that conveys positional information indicating “posterior”
LE 47-27
Anterior
Posterior
New ZPA
Hostlimbbud
ZPA
Donorlimbbud
• Signal molecules produced by inducing cells influence gene expression in cells receiving them
• Signal molecules lead to differentiation and the development of particular structures